1. Field of the Invention
The invention relates to a reflective film for thermal control, and more particularly to thermal control in high altitude environments.
2. Description of the Related Art
Polyimides are an important class of polymeric materials and are known for their superior performance characteristics. These characteristics include high glass transition temperatures, good mechanical strength, high Young's modulus, good UV durability, and excellent thermal stability. Most polyimides are comprised of relatively rigid molecular structures such as aromatic/cyclic moieties.
As a result of their favorable characteristics, polyimide compositions have become widely used in many industries, including the aerospace industry, the electronics industry and the telecommunications industry. In the electronics industry, polyimide compositions are used in applications such as forming protective and stress buffer coatings for semiconductors, dielectric layers for multilayer integrated circuits and multi-chip modules, high temperature solder masks, bonding layers for multilayer circuits, final passivating coatings on electronic devices, and the like. In addition, polyimide compositions may form dielectric films in electrical and electronic devices such as motors, capacitors, semiconductors, printed circuit boards and other structures. Polyimide compositions may also serve as an interlayer dielectric in both semiconductors and thin film multichip modules. The low dielectric constant, low stress, high modulus, and inherent ductility of polyimide compositions make them well suited for these multiple layer applications. Other uses for polyimide compositions include alignment and/or dielectric layers for displays, and as a structural layer in micromachining applications.
In the aerospace industry, polyimide compositions have been used for optical applications as reflective membranes. In application, a polyimide composition is secured by a metal (often aluminum, copper, or stainless steel) or composite (often graphite/epoxy or fiberglass) mounting fixture that secures the polyimide compositions. Such optical applications may be used in space, where the polyimide compositions and the mounting fixture are subject to repeated and drastic heating and cooling cycles in orbit as the structure is exposed to alternating periods of sunlight and shade. Satellites can remain in orbit for extended periods, so the materials used should survive as long as the satellite remains functional.
One important factor of a material in space is its service temperature. Generally, there are three types of heat transfer: conductive heat transfer where heat passes to or from a first object to a second object touching the first object; convective heat transfer where heat passes from a first object to a gas, such as the atmosphere; and radiant heat transfer, where heat passes to or from an object through electromagnetic radiation, such as infrared (IR) light. Radiant heat transfer controls the temperature of an object in space. Radiant heat transfer can also be significant for objects in high altitude environments, because the atmosphere for convective heat transfer is thin and no objects are present for conductive heat transfer. “High altitude environment” is defined to include altitudes above 20,000 feet, the stratospheric near-space environment, and space.
Polyimide polymers are subject to rapid degradation in a highly oxidizing environment, such as an oxygen plasma or atomic oxygen [AO], as are most hydrocarbon- and halocarbon-based polymers. AO is present in low earth orbit [LEO], so many spacecraft experience this highly oxidizing environment. The interactions between the oxidizing environment and the polymer material can erode and reduce the thickness of the polymer material. To protect from erosion, protective coatings including metals, metal oxides, ceramics, glasses, and other inorganic materials can be applied as surface treatments to polyimides subjected to the oxidizing environment.
While these coatings are effective at minimizing the oxidative degradation of the underlying material, they often experience cracking from thermal and mechanical stresses, mechanical abrasion, and debris impact. After cracking, the protective surface is compromised and the underlying polymeric material can be degraded from additional exposure to the oxidizing environment. Therefore, the availability of polymers which are able to resist AO degradation is very desirable.
Oligomeric silsesquioxanes [OS] can be incorporated into a polyimide matrix to improve the durability of polyimides in these environments. Polyimides with incorporated OS demonstrate excellent resistance to AO degradation prevalent in LEO environments. Polyimides with incorporated OS provide additional benefits as well. Polyhedral OS are referred to by the trademark POSS™, and are a common form of OS.
Rockets, aircraft, other lift engines, and lighter than air gases are often used to carry a cargo into space or other high altitude environments. Much of the total mass of a space rocket is the fuel used to propel the rocket into space, and the mass of any cargo carried into high altitude environments is limited. Cargo carried into high altitude environments is typically limited to a certain size and weight to fit within the confines and capabilities of the vehicle carrying the cargo. The cost per unit weight of delivering a vehicle, cargo, and payload into high altitude service is high, so many efforts are made to reduce the total weight. Reductions in the total volume occupied by the cargo are also beneficial. The various components of a vehicle, cargo, and/or payload should be able to withstand the harsh high altitude environment for the expected life. Reducing the cargo weight can save costs, or allow for additional components to be included with the vehicle. Efforts to find new materials or combinations of materials which can withstand the high altitude environment and which can reduce the weight and volume of the vehicle, cargo, and/or payload continue. Materials which can withstand high altitude environments may also be beneficial in other environments, such as certain industrial applications.